Towards an improved understanding of wind turbine wakes in complex terrain

In this thesis, we explored the effect of certain terrain-induced flow phenomena on the development of wind turbines sited in complex terrain. A combined experimental and analytical approach is used to study wind turbine wakes in different types of complex terrain. The thesis is divided in three parts: wind turbine wakes on escarpments, wind turbine wakes under pressure gradient and wind turbines in urban environments.

In the first part of the thesis, a wind turbine is sited close to the edge of an escarpment. The shape of the windward side of the escarpment is varied in order to investigate its effect on the wake of the wind turbine. Stereoscopic and tomographic particle-image velocimetry is used to capture the flow. Five different escarpment shapes are considered varying between forward facing steps and ramp shapes. The wake of turbine sited on forward facing step escarpment shows faster recovery due to higher turbulence, where the curvature of the leading edge is shown to significantly reduce the flow separation, whereas, the ramp-shaped escarpments show a relatively slower recovery. The growth and meandering of the wake, as well as, the vortices shed from the turbine are affected by the escarpment shape.

The effect of wind direction on the flow over a cliff and its interaction with wind turbine wake is then explored. The flow becomes increasingly three-dimensional with the increase in the obliqueness of the wind direction. This has an effect on the recovery, shape and deflection of the wind turbine wake.

In the second part of the thesis, the effect of terrain-induced pressure gradient on the wake of a wind turbine is investigated. An analytical framework to model the wake velocity deficit is proposed and validated with the measurements of wakes on escarpments. This is followed by a systematic study on the wake of a turbine exposed to a range of pressure gradients. The wake recovery, growth rate and transition from near to far wake are analyzed as a function of the imposed pressure gradient. The analytical framework is further developed and validated with the experiments.

A study of the cumulative wake of multiple turbines exposed to pressure gradient is performed. In this context, the analytical framework is applied to model the cumulative wake of multiple wind turbines and compared with an existing method. The new method is shown to work better than the existing method and agree well with the experiments.

In the third part of the thesis, roof-mounted wind turbines in an urban environment are studied. The effect of roof edge shape on the power and wake of a roof mounted wind turbine is investigated. This study shows that round edged roofs are favorable, whereas, roofs with solid fences are unfavorable for wind turbine power and wake. Roof boundary fences are, however, important from a safety perspective. Therefore, a parametric study on the effect of fence shape on wind turbine performance is performed. Fences which are angled inwards o